“Joni is running for United States Senate in Iowa and is the type of tough, no nonsense conservative that we need in Washington” said Rubio, R-Fla.

Ernst is locked in a tight race with businessman Marc Jacobs for the GOP nomination. The winner is expected to face Democratic Rep. Bruce Braley. Ernst has also been backed by 2012 presidential nominee Mitt Romney and 2008 vice presidential nominee Sarah Palin.

She got national attention for her ad last month, where she explains how “I grew up castrating hogs on an Iowa farm. So when I get to Washington, “I’ll know how to cut pork.”

Iowa is a traditionally closely contested state, and Republicans see a chance to pick up a Senate seat there this year. Veteran Democrat Tom Harkin is retiring.

“Although there are other good candidates running, it is clear that Joni is not only the kind of small government conservative we need in Washington,” Rubio said. “She’s exactly the kind of no nonsense candidate we need to turn a blue state red and pick up a critical Senate seat.”

Though endorsing Republicans in primaries not that unusual, this is a first for Rubio and his Reclaim America Political Action Committee.